In a typical consumable electrode arc welding, welding is performed by feeding a welding wire as a consumable electrode at a constant feeding rate and generating an arc between the welding wire and base material. In the consumable electrode arc welding, both the welding wire and the base material are mostly placed in a welding state in which a short circuit state and an arc generation state are alternately repeated.
In order to further improve welding quality, there has been proposed a welding method of alternating feeding of a welding wire between forward feeding and reverse feeding cyclically.
FIG. 3 is a waveform diagram of the welding method in which the forward feeding and the reverse feeding are repeated cyclically as to the feeding rate. (A) of this figure shows a waveform of a feeding rate Fw, (B) of this figure shows a waveform of a welding current Iw and (C) of this figure shows a waveform of a welding voltage Vw. Hereinafter explanation will be made with reference to this figure.
As shown in (A) of this figure, in the feeding rate Fw, an upper side and a lower side than 0 represent a forward feeding period and a reverse feeding period, respectively. The forward feeding represents feeding of the welding wire in a direction approaching the base material, whilst the reverse feeding represents feeding of the welding wire in a direction separating from the base material. The feeding rate Fw has a waveform which changes sinusoidally and shifts on the forward feeding side. Thus as an average value of the feeding rate Fw is positive, the welding wire is fed forwardly in average.
As shown in (A) of this figure, the feeding rate Fw is 0 at a time t1. A period from the time t1 to a time t2 corresponds to a forward feeding acceleration period. The feeding rate is the maximum value of the forward feeding at the time t2. A period from the time t2 to a time t3 corresponds to a forward feeding deceleration period. The feeding rate is 0 at the time t3. A period from the time t3 to a time t4 corresponds to a reverse feeding acceleration period. The feeding rate is the maximum value of the reverse feeding at the time t4. A period from the time t4 to a time t5 corresponds to a reverse feeding deceleration period.
Short circuit between the welding wire and the base material occurs mostly before or after the maximum value of the forward feeding at the time t2. This figure shows a case where the short circuit occurs at a time t21 during the forward feeding deceleration period after the maximum value of the forward feeding. If the short circuit occurs at the time t21, the welding voltage Vw rapidly reduces to a short-circuit voltage value of a few volts as shown in (C) of this figure, and the welding current Iw also reduces to an initial current value of a small current value as shown in (B) of this figure. Thereafter the welding current Iw increases with a predetermined inclination. When the welding current reaches a predetermined peak value, the welding current is maintained at this value.
As shown in (A) of this figure, from the time 13, as the feeding rate Fw is placed in the reverse feeding period, the welding wire is reversely fed. The short circuit is released by this reverse feeding, and an arc is regenerated at a time t31. The arc is regenerated mostly before or after the maximum value of the reverse feeding at the time t4. This figure shows a case where the arc is generated at the time t31 during the reverse feeding acceleration period before the maximum value of the reverse feeding.
If the arc is regenerated at the time t31, the welding voltage Vw increases rapidly to an arc voltage value of several tens of volts as shown in (C) of this figure. As shown in (B) of this figure, according to detection control of a narrow part of a droplet as a precursory phenomenon of arc regeneration, the welding current Iw rapidly reduces from a time earlier than the time t31 by several hundreds of μs and becomes a small current value at the arc regeneration time t31. In this respect, if the narrow part is formed at the droplet, a current path become narrow, and hence a resistance value or the welding voltage value between the welding wire and the base material increases. The narrow-part detection is performed by detecting this increase.
As shown in (A) of this figure, from the time t31 to the time t5, the feeding rate Fw is placed in a reverse feeding state. An arc length becomes long during this period. As shown in (B) of this figure, during the period from the time t31 to the time t5, the welding current Iw increases with a predetermined inclination, then reaches a predetermined first welding current value and maintains this value until a predetermined period elapses after the arc regeneration (time t31). Thereafter a second welding current smaller than the first welding current flows until a time 61 where the next short circuit occurs.
As shown in (A) of this figure, the feeding rate Fw is placed in the forward feeding period from the time t5 and reaches a forward feeding peak value at a time t6. Then the next short circuit occurs at a time t61. During a period from the time t5 to the time t61, the welding voltage Vw reduces gradually as shown in (C) of this figure, and the welding current Iw also reduces gradually as shown in (B) of this figure.
As described above, a cycle of the short circuit and the arc substantially coincides with a cycle of the forward feeding and the reverse feeding as to the feeding rate. That is, according to this welding method, the cycle of short circuit and arc can be set to a desired value by suitably setting the cycle of forward feeding and reverse feeding as to the feeding rate. Thus if this welding method is implemented, the cycle of short circuit and arc can be suppressed in its variance and made substantially constant. Consequently the welding can be performed with a small generation amount of spatter and good bead appearance.
However, in the welding method of repeating the forward feeding and the reverse feeding as to the feeding rate, there arises a case where the short circuit does not occur at the suitable timing due to disturbance such as irregular movements of a molten pool and a distance between a power supply tip and the base material, and a change of a welding posture. In this case, as the cycle of short circuit and arc does not synchronise with the cycle of forward feeding and reverse feeding, the cycle of short circuit and arc varies. A method of restoring this asynchronous state to an original synchronous state is disclosed in patent document 1.
According to an invention of the patent document 1, in a case where short circuit does not occur until a feeding rate reaches a predetermined feeding rate during feeding-rate deceleration in forward feeding of a welding wire, cyclical change of the feeding rate is stopped and the feeding rate is controlled to a constant value of a first feeding rate. When short circuit occurs in the forward feeding at the first feeding rate, reduction of the feeding rate from the first feeding rate is started and cyclical change of the feeding rate is restarted to perform the welding. Consequently the asynchronous state is intended to be restored to the synchronous state.